Microelectromechanical system (MEMS) devices have the potential for great impact on the communications industry. MEMS RF switches, oscillators (resonators), filters, varactors, and inductors are a few of the devices that could replace large and relatively expensive off-chip passive components. It is even possible that the introduction of these types of MEMS devices, particularly resonators and filters, into analog and mixed-signal integrated circuits could dramatically alter the architecture of current wireless communication devices. Key to such advancements is the ability to monolithically integrate MEMS RF components with integrated circuit technologies to realize cost, size, power, and performance benefits.
A significant portion of the cost of MEM devices can be in the packaging, which can account for more than half of the total product cost for MEM devices. MEM devices typically have moving micro parts which must be protected from small particles. It is therefore desirable to package the MEM devices at wafer scale, prior to dicing the parts which creates many fine particles. To achieve high reliability operation of MEM devices such as switches, such devices must be hermetically sealed with an inert gas. For other MEM devices such as resonators, vacuum packaging is required to prevent reduction in the resonance quality factor by air damping.
A number of approaches have been proposed for MEM device encapsulation, where the most common methods involve bonding a second substrate over the MEM device to form a sealed cavity. For example, a hermetic seal can be formed by metal bonding a thin silicon wafer containing vertical electrical feed-throughs to the MEM device (see Y-K. Park et al., Innovation ultra thin packaging for RF-MEMs Devices, The 12th Int. Conf. on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003 pp. 903-906). In addition, K. Totsu et al., Vacuum sealed ultra miniature fiber-optic pressure sensor using white light interferometry, The 12th Int. Conf. on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003 pp. 931-934, describes the formation of a Fabry-Perot cavity between the end of an optical fiber and a silicon dioxide membrane sealed in vacuum using a solder. Moreover, D. Briand et al., Metal to glass anodic bonding for microsystems packaging, The 12th Int. Conf. on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003 pp. 1824-1827, describes the anodic bonding of metal sheets, such as Invar, Kovar, and Alloy 42, to Pyrex for packaging micro-fluidic systems. In addition, T. Itoh et al., Room temperature vacuum sealing using surface activated bonding method, The 12th Int. Conf. on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003 pp. 1828-1831, describes surface activated bonding between Si/Si and Si/Cu surfaces. However, when an MEM device is integrated with an integrated circuit, the above methods are not compatible with designs intended for flip-chip packaging (i.e. C4 mini solder balls) and assume that the MEM resonator or filter is exposed on the top surface of the chip.
Another encapsulation method which has been proposed uses surface micro machining to form a local cavity over the MEM device. With the approach, sacrificial and capping layers are formed over the device, small etch holes are opened in the cap layer, the sacrificial layer is etched away, and then the holes are filled by growth or deposition of additional blanket films. Typically, these holes are subsequently pinched-off using a plasma deposition process. This approach offers a simple low cost means of encapsulation but with some risk of MEM device impairment by undesired deposition of material on the active device during pinch-off processing. This method is also limited to pressures of a few Torr or more in the sealed structure (i.e. the pressure at which the plasma deposition takes place) if a PECVD or CVD technique are used to perform the sealing operation.
For example, H. Stahl et al., Thin film encapsulation of acceleration sensors using polysilicon sacrificial layers, The 12th Int. Conf. on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003 pp. 1899-1902, describes the use of a poly silicon sacrificial layer and a polysilicon cap layer. The poly silicon sacrificial layer is etched using ClF3 or XeF2 and the vent holes are sealed by depositing a non-conformal PECVD oxide. Note that the ambient conditions of the device are determined by the sealing process used and the PECVD oxide can be inadvertently deposited on the active device. In addition, W-T. Park et al., Wafer-scale film encapsulation of micromachined accelerometers, The 12th Int. Conf. on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003 pp. 1903-1906, describes the use of a silicon dioxide sacrificial layer and an epitaxial polysilicon cap layer. The sacrificial layer is etched using vapor HF and the vent holes are sealed by depositing a non-conformal low temperature oxide layer. Again, with such method, the ambient conditions of the device are determined by the deposition conditions of the sealing process used and the sealing material can be inadvertently deposited on the active device.
U.S. Patent Application publication US2002/0197761 A1, published Dec. 26, 2002, describes methods for releasing an MEM structure and describes a number of packaging methods which can be used including wafer bonding using anodic bonding, metal eutectic bonding, fusion bonding, epoxy bonding or other bonding processes where the preferred bonding method is using IR or UV epoxy. Also described is dispensing an epoxy, polymer, or other adhesive in a gasket region. Moreover, U.S. Patent Application publication US2003/0138986 A1, published Jul. 24, 2003, describes encapsulating release structures where access holes or trenches can be sealed by any number of methods including sputtering, CVD, PECVD, or spin on glass methods. The access trenches can be sealed with any number of materials including metals, polymers and ceramics. Preferably, they are sealed by sputtering a layer of Al over the access trenches and capping layer. As noted above, a disadvantage of these methods of sealing the access holes is that the ambient conditions of the device are determined by the deposition conditions of the sealing process used.
From a general perspective, a variety of thin film processing techniques, or combinations thereof, can be used to seal small vent holes. Vacuum evaporation of materials is generally the lowest pressure deposition process and may be the most effective technique to obtain a low pressure seal. However, evaporation processes result in a line of sight coverage and films would have to be deposited in a special configuration to preferentially deposit the film on the vent side wall to promote closing of the vent hole. Additionally, any MEM component exposed to the evaporative flux will receive a significant deposition of material that can alter the performance of the MEM device. For example, adding mass to a micro mechanical resonator (or beam) will change the operational frequency of the resonator or cause stress related distortions of the beam. By way of further example, an MEM switch device could be rendered inoperative if coatings get onto the switch contact and cause shorts or opens. High pressure thin film processes, CVD, PECVD, and LPCVD as examples, typically work best for sealing a vent hole and providing the least amount of material coatings within the cavity. The drawbacks of these techniques are the high pressure they would seal in the cavity and because they are generally performed at high temperatures (>200 degrees C.) all surfaces within the cavity are prone to some coating from the gaseous precursor, albeit perhaps a very thin coating.
An intermediate pressure process is sputter deposition. With most sputtering processes, a minimal pressure of about 1 mT of an inert gas, typically argon, is needed to establish a plasma for sputtering. This represents the nominal minimal pressure obtainable. As a result common thin film deposition processes applied to seal small vent holes is limited by both the attainable vacuum level and the potential for detrimentally coating the MEM devices.